This disclosure relates generally to porous materials, and more particularly to producing oxides and other porous or partly-porous compositions and structures having enhanced mechanical strength.
Porous materials with feature sizes expressible roughly in terms of nanometers or smaller comprise an important class of materials useful in an ever-increasing range of applications. Chief among these applications are membranes (U.S. Pat. No. 6,536,604, Brinker, et al.), sensors (U.S. Pat. No. 5,885,843, Ayers and Hunt), display devices (U.S. Pat. No. 6,329,748, Kastalsky, et al.), and, notably, microelectronic devices, where such materials show promise as low dielectric constant materials (U.S. Pat. No. 5,789,819, Gnade, et al.). However, many seemingly useful materials do not exhibit the necessary mechanical strength required by the aforementioned applications. This results from the fact that many desirable properties, such as high permeability or low dielectric constant are maximized only when the material possesses a relatively high void fraction, or porosity. High porosity generally leads to lower-strength materials, relative to fully dense materials of similar compositions, and typically precludes the use of such materials in practice. Increasing the mechanical strength of porous materials is most easily accomplished by simply lowering their porosity. However, this often counteracts many of the beneficial properties of higher porosity analogs. Therefore there is an important need for materials with a favorable combination of relatively high porosity and good mechanical properties.
Common terms used in the art, and also used herein, to describe the microstructure of porous materials are microporous, referring to a material possessing pores between 0.3 and 1.0 nm in diameter, mesoporous, for pores between 1.0 and 50 nm, and macroporous for pores greater than 50 nm. Porosity refers to the volume fraction of the material occupied by a fluid or gaseous phase, and is commonly reported as a percentage of the total volume.
Many commonly occurring mesoporous materials exhibit a microstructure formed by the linking of small (1-10 nm diameter) particles into a three-dimensional network spatially conterminous with an interconnecting open-pore network. Such structures are commonly formed by sol-gel processing, a technique well-known to those skilled in the art. The particles mentioned above may occur in a wide variety of shapes, including spheres, rods, platelets, polygons, as well as irregular shapes. The points of contact between these particles result in material deficiencies that lead to weaknesses in their bulk structures.
There exist few methods which are effective in strengthening the solid network of a mesoporous material without adversely affecting its desirable properties. One such method is Ostwald ripening. See Brinker and Scherer, Sol-Gel Science: The Physics and Chemistry of Sol-Gel Processing, Academic Press, Chapter 3.3 (1989). In that process, within certain pH ranges, material is dissolved from solid surfaces with a positive (concave) radius of curvature into the surrounding pore fluid, and redeposited on surfaces with a negative (convex) radius of curvature. However, this process is slow. Additionally, as the material which serves to reinforce weak points in the network must be obtained at the sacrifice of other parts of the same network, there is a finite amount of strength improvements that may be obtained in this way.
Another approach has been demonstrated by Haereid, et al. (Journal of Non-Crystalline Solids, vol. 186, pages 96-103 [1995]). Haereid, et al. strengthened a wet silica gel by reinforcement of the solid network using a similar alkoxide precursor to that which formed the original gel. Although this approach may toughen the solid network, it does so by decreasing the porosity of the material significantly. This may have undesired side effects, such as where decreasing the porosity of a mixture by adding silica would have an undesirable effect on the mixture's overall dielectric constant.
Therefore, there remains a distinct need for materials and procedures capable of significantly enhancing the physical and mechanical properties of porous solid networks without significantly decreasing the material's porosity, or otherwise adversely affecting its desirable properties.
It should be noted that useful materials such as fullerenes have found uses in the general field of porous materials, for example as a sacrificial templating material (U.S. Pat. No. 5,744,399, Rostoker, et al.), or as blocking groups for preventing material shrinkage (U.S. Pat. No. 6,277,766, Ayers). However, such materials have had limited use due to their poor solubility and/or (historical) high cost.